Pulps may vary considerably in their color after pulping, depending on the type of wood, processing method, and other factors. For many types of pulp, bleaching is required to obtain a pulp having the desired level of brightness. Brightness is measured by measuring the reflectance in the blue range (457 nm) using magnesium oxide as a standard (100% brightness).
The most used bleaching agent for pulps is hydrogen peroxide because of its low cost and its effectiveness in bleaching. Many different pulp bleaching processes are known which employ hydrogen peroxide as the bleaching agent. A few examples of such processes can be found in U.S. Pat. No. 4,798,652 and 4,732,650. From these patents it can be seen that the process for bleaching wood pulp often involves a pretreatment step and a bleaching step.
From U.S. Pat. No. 4,798,652 it is known to use diethylene triamine pentaacetic acid (DTPA) in the pretreatment of pulps prior to bleaching them with hydrogen peroxide. The pretreatment is generally carried out at a pulp consistency of less than 5% and the chelating agent is preferably a relatively powerful complexing agent.
From U.S. Pat. No. 4,732,650 it is known to use chelating agents in the pretreatment step prior to bleaching the pulp with hydrogen peroxide. Typical chelating agents used in the pretreatment step include ethylene diamine tetraacetic acid (EDTA), DTPA, triethylene tetramine hexaacetic acid (TTHA) and N-hydroxyethylene diamine triacetic acid (HEDTA).
The article, "Chelating Agents in the Pulp and Paper Industry," Hart, J. Roger, Tappi Journal, Vol. 64, No. 3, pp. 43-44, (March 1981) discusses the role of chelating agents in the bleaching of pulps. This article points out that chelating agents may be used both in the pretreatment of pulp prior to the bleaching step, and as an additive to the bleaching liquor during the bleaching step to stabilize hydrogen peroxide. EDTA and DTPA are mentioned as the preferred chelates.
Another article relating to this subject is, "The Effect of DTPA on Reducing Peroxide Decomposition," Bambrick, D. R., Tappi Journal, Vol. 66, No. 6, pp. 96-100, (June 1985) which gives a detailed discussion on the role of DTPA in hydrogen peroxide bleaching of wood pulp in combination with silicates and magnesium compounds. The article, "Hydrogen Peroxide: Stabilization of Bleaching Liquors," Kutney, G. W., Pulp & Paper Canada, Vol, 86, No. 12, pp. 182-189, (1985) gives a detailed summary of a wide variety of compounds which have been used to stabilize hydrogen peroxide bleaching liquors and thereby obtain a higher pulp brightness. Several different chelates are mentioned including EDTA, DTPA and nitrilotriacetic acid (NTA).
Finally, the article, "Improving Hydrogen Peroxide Bleaching of Mechanical Pulp: The Effect of Silicate Dose and Other Additives," Burton, J. T., et al., Pulp & Paper-Canada, Vol. 86, No. 6, pp. 144-147, (1987) gives some further information on the effects of different additives on the bleaching of pulp with hydrogen peroxide.
Among the chelates employed are DTPA and diethylene triamine pentamethylenephosphonic acid (DTMPA, also referred to as DTPMPA). The ultimate goal of using these additives in the bleach liquor is to replace the silicate additives.
From these last two articles it is apparent that it is desirable to reduce the content of silicates in the present pulp bleaching systems. Further, the chelates used in pulp bleaching suffer from the disadvantage that they are substantially non-biodegradable. One goal of present research in the pulp bleaching field is to develop more environmentally friendly bleaching systems by, for example, reducing the silicate content, providing more biodegradable bleaching additives and/or enhancing the activity of the bleaching liquor to thereby use less bleaching liquor to achieve the same level of brightness.
Another article which tends to indicate the desirability of reducing the amount of silicates used in pulp treatment is, "Chelant Optimization in De-Inking Formulation," Mathur, I., Pulp & Paper Canada, 94:10, pp. 55-60 (1993). This article mentions that EDTA, HEDTA, DTPA and DTPMPA are employed in pulp deinking. DTPA is given as the chelate of choice for the deinking process.
It is accordingly the primary object of the present invention to provide an effective pulp bleaching system which is more environmentally friendly than present, commercial bleaching systems employing EDTA and/or DTPA chelating agents with or without silicates.
This and other objects and advantages of the invention will be apparent from the summary and detailed description which follow.